Back to EveryPatent.com
| United States Patent |
5,754,090
|
|
Arensmeier
|
May 19, 1998
|
Thermostat having a temperature sensing element which includes a member
having a negative coefficient of thermal expansion
Abstract
A thermostat includes a valve for controlling gas flow, an actuator for
controlling operation of the valve, and a temperature sensing element. The
temperature sensing element comprises two members, one of which is made of
a material having a positive coefficient of thermal expansion and the
other of which is made of a material having a negative coefficient of
thermal expansion. Each member effects movement of the actuator in a first
direction upon sensing a decrease in temperature to effect opening of the
valve and in a second direction upon sensing an increase in temperature to
effect closing of the valve.
| Inventors:
|
Arensmeier; Jeffrey N. (St. Louis, MO)
|
| Assignee:
|
Emerson Electric Co. (St. Louis, MO)
|
| Appl. No.:
|
684144 |
| Filed:
|
July 19, 1996 |
| Current U.S. Class: |
337/394; 337/123; 337/382 |
| Intern'l Class: |
H01H 037/48 |
| Field of Search: |
337/123,382,392,393,394
219/449
|
References Cited
U.S. Patent Documents
| 2856489 | Aug., 1958 | Bletz | 337/394.
|
| 3447746 | Jun., 1969 | Visos | 236/21.
|
| 3691501 | Sep., 1972 | Katchka et al. | 337/393.
|
| 5294907 | Mar., 1994 | Katchka | 337/394.
|
Primary Examiner: Picard; Leo P.
Assistant Examiner: Gandhi; Jayprakash N.
Attorney, Agent or Firm: Becker, Sr.; Paul A.
Claims
I claim:
1. In a thermostat for controlling temperature of water in a gas-fired
water heater,
a valve for controlling flow of gas to a burner in the water heater;
an actuator for controlling operation of said valve;
a flange for connecting the thermostat to said water heater;
a first temperature responsive member being tubular and made of a material
having a positive coefficient of thermal expansion having an open end
connected to an internal bore of said flange and having a closed end
extending into said water of said water heater; and
a second temperature responsive member made of a material having a negative
coefficient of thermal expansion slidably mounted within said first
temperature responsive member having a first end in contact with said
closed end of said first temperature responsive member and having a second
end in contact with said actuator.
2. The thermostat claimed in claim 1 wherein said material of said first
temperature responsive member comprises copper and said material of said
second temperature responsive member comprises ceramic.
3. The thermostat claimed in claim 2 wherein said ceramic has a composition
of LiAlSiO.sub.4.
4. The thermostat claimed in claim 1 wherein said connection of said open
end of said first temperature responsive member to said internal bore of
said flange comprises a swaged joint.
5. The thermostat claimed in claim 1 wherein said closed end of said first
temperature responsive member comprises a plug which is press-fitted into
said first temperature responsive member.
6. The thermostat claimed in claim 1 wherein said second temperature
responsive member is tubular and wherein the thermostat further includes a
limit switch located inside of said second temperature responsive member.
Description
BACKGROUND OF THE INVENTION
This invention relates to thermostats utilized in water heaters, and
particularly to an improved construction of the temperature sensing
element therein.
Typically the thermostat in domestic gas-fired water heaters utilizes a
so-called "rod and tube" sensing element connected to the thermostat
housing. Such sensing element typically comprises an invar rod and a
copper tube. The copper tube has a relatively high coefficient of thermal
expansion, and the invar rod has a relatively low coefficient. One end of
the invar rod is connected to the copper tube. The other end of the invar
rod is moved toward or away from the thermostat housing primarily in
response to the expansion and contraction of the copper tube as the copper
tube responds to the changes in water temperature. The invar rod also
expands and contracts in response to the changes in water temperature, but
to a lesser degree because of its low coefficient. However, the expansion
and contraction of the invar rod causes movement of its free end in a
direction opposite to that caused by the copper tube. The resulting
movement toward and away from the thermostat housing is transmitted to
valve actuating means in the thermostat housing to effect opening and
closing of a gas valve member in the thermostat which controls the flow of
gas to the burner. While such prior art thermostats are generally
satisfactory, the temperature differential, which is the difference in
water temperature between the temperatures at which the burner turns on
and off, is greater than desired.
SUMMARY OF THE INVENTION
An object of this invention is to provide a generally new and improved
thermostat having a temperature sensing element which enables a lower
temperature differential.
In the preferred embodiment, a thermostat comprises a temperature sensing
element comprising a first member having a positive coefficient of thermal
expansion and a second member having a negative coefficient of thermal
expansion. Each member effects movement of a valve actuator in a first
direction upon sensing a decrease in temperature to effect opening of the
valve, and in a second direction upon sensing an increase in temperature
to effect closing of the valve. Such construction results in a temperature
differential that is considerably less than that obtained in the
conventional rod and tube construction.
The above mentioned and other objects and features of the present invention
will become apparent from the following description when read in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The single FIGURE in the drawings is a partial cross-sectional view of the
thermostat constructed in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, the thermostat of this invention comprises a
housing indicated generally at 10 and a temperature sensing element
indicated generally at 12. Sensing element 12 is connected to housing 10
by a plurality of screws 14.
Housing 10 includes a valve 16 biased towards a valve seat 18 by a spring
19. A valve stem 20 is connected at one end to valve 16. The other end of
stem 20 extends through a bore 22 into a circular cavity 24. Mounted
within cavity 24 are a rigid disc 26 and a clicker spring 28. Clicker
spring 28 has a normal free-form position in which it is concave with
respect to stem 20. The clicker spring 28 is positioned between a wire
loop 30, which engages the clicker spring 28 on its normally-concave side
near its peripheral edge, and an annular knife-edge 32 of slightly smaller
diameter formed on the side of disc 26 which faces spring 28. A slight
movement of disc 26 in the direction of clicker spring 28 causes clicker
spring 28 to pass through a planar shape, with a snap action, to an
opposite form wherein it is convex with respect to stem 20. When clicker
spring 28 passes through the planar shape, central leg portions 34 thereof
engage stem 20, as shown in the drawing, causing valve 16 to be lifted off
valve seat 18 against the bias of spring 19. Gas can then flow from a
source (not shown), past valve seat 18 and to a gas burner (not shown).
The side of disc 26 which faces sensing element 12 is provided with a
hemispherical boss 36 which cooperates with a recess 38 in one end of a
lever 40. Lever 40 is pivoted at its other end on the tip of an adjusting
screw 42. Screw 42 is adjustable by means of a knob 44. Lever 40 is also
provided with an intermediate hemispherical boss 46.
The construction of housing 10 described above is well known in the art and
it is believed that it has been described herein in sufficient detail for
purposes of the present invention. If more detail is desired, reference
may be had to Visos, U.S. Pat. No. 3,447,746, this patent hereby being
incorporated into this disclosure by this reference.
Temperature sensing element 12 includes a brass flange 48 having one end 50
thereof attached by screws 14 to housing 10 and the other end 52 being
externally threaded. An internal bore 54 of flange 48 is provided with a
plurality of shallow grooves 56. One end of a copper tube 58 is secured
inside internal bore 54 of flange 48 by a swaged joint obtained by forcing
those portions of copper tube 58 adjacent grooves 56 into the grooves 56
thereby producing peripheral projections 60 on copper tube 58 which
cooperate with grooves 56 to provide a rigid and water-sealing connection
of copper tube 58 to flange 48. Copper tube 58 has a positive coefficient
of thermal expansion.
Slidably mounted within copper tube 58 is a tube 62 made of a ceramic
material having a negative coefficient of thermal expansion. Tube 62 is
sandwiched between a plug 64 at one end and an actuator 66 at its other
end. Plug 64 is made of a metal, such as brass, having a desired level of
heat conductivity. Plug 64 is secured inside copper tube 58 in a press-fit
manner so as to prevent plug 64 from moving relative to copper tube 58
when force is applied to plug 64 by tube 62. Solder is applied at 68 to
ensure that water will not enter into copper tube 58. Actuator 66 is made
of a metal, preferably steel. Actuator 66 has a large diameter end 70
which abuts against one end of tube 62 and a small diameter end 72 which
abuts against boss 46 of lever 40.
Passing through a slot 74 in flange 48 and a slot 76 in actuator 66 are two
leads 78 and 80 which connect external circuitry (not shown) to a
high-temperature limit switch 82 which is located inside of tube 62. If,
for any reason, the sensed water temperature should rise above a
predetermined value, switch 82 will respond to such temperature by
interrupting the external circuitry and thereby effecting the closing of a
safety valve (not shown) in housing 10 upstream from and in series flow
relationship with the gas flow controlled by valve 16.
In operation, the thermostat of this invention is assembled to a water
heater (not shown) in a well known arrangement wherein flange 48 is
threadedly connected to a threaded nipple (not shown) in the water heater
so that sensing element 12 extends through a central opening of the
threaded nipple and is thereby immersed in the water of the water heater.
A desired set point temperature is selected by knob 44 which is connected
to adjusting screw 42 which, in turn, adjusts the pivot point of lever 40.
As the water temperature drops, copper tube 58 contracts and ceramic tube
62 expands. Because ceramic tube 62 is sandwiched between plug 64 and
actuator 66 and because plug 64 is rigidly connected to copper tube 58,
contraction of copper tube 58 and expansion of ceramic tube 62 cause
actuator 66 to be moved to the left as viewed in the drawing, thereby
causing movement of lever 40 around its pivot point on the end of
adjusting screw 42 which, in turn, causes movement of disc 26. Upon
sufficient movement to the left, actuator 66 effects sufficient movement
of lever 40 and disc 26 to cause clicker spring 28 to snap through its
planar shape and effect opening of valve 16. With valve 16 open, gas then
flows to the burner to effect heating of the water.
As the water temperature subsequently rises, copper tube 58 expands and
ceramic tube 62 contracts, enabling actuator 66 to be moved to the right
by the biasing forces of clicker-spring 28 and spring 19. Upon sufficient
movement to the right, actuator 66 enables sufficient movement of lever 40
and disc 26 to enable clicker spring 28 to snap through its planer shape
and effect closing of valve 16. With valve 16 closed, gas flow to the
burner is shut off.
The temperature differential of the thermostat of this invention is
considerably less than the differential of a typical prior art thermostat
utilizing the conventional invar rod and copper tube construction of the
sensing element. For example, a typical differential of such a prior art
thermostat is approximately 22.degree. F.; the differential of a
thermostat of this invention was found to be approximately 14.degree. F.
It is believed that this reduction in differential is due to the
utilization of a temperature sensing member having a negative coefficient
of thermal expansion. Specifically, in the conventional rod and tube
construction, both the copper tube and the invar rod have positive
coefficients of thermal expansion. While the coefficient of the invar rod
is quite small, approximately one-tenth of the coefficient of copper, the
invar rod causes movement of the switch actuating means in a direction
opposite to that provided by the copper tube, causing the temperature
differential to be greater than it would be due solely to the movement of
the switch actuating means caused by the copper tube. In the thermostat of
the present invention, ceramic tube 62, having a negative coefficient of
thermal expansion, causes movement of actuator 66 in the same direction as
that provided by copper tube 58, causing the differential to be less than
it would be due solely to the movement of actuator 66 caused by copper
tube 58.
In the preferred embodiment, tube 62 is made of a ceramic having a
composition of LiAlSiO.sub.4 (.beta.-eucryptite) having a negative
coefficient of thermal expansion of approximately 0.0000042
in/in/.degree.F.(inch per inch per degree Fahrenheit.) In a working
embodiment, ceramic tube 62 is approximately four inches long with an
outside diameter of 0.540 inches and an inside diameter of 0.390 inches,
and copper tube 58 is approximately five inches long with an outside
diameter of 0.625 inches and an inside diameter of 0.561 inches. Copper
tube 58 has a positive coefficient of thermal expansion of approximately
0.000010 in/in/.degree.F.
It is to be understood that other ceramic materials can possibly be used
for tube 62. Examples of such other ceramic materials include LiAlSi.sub.2
O.sub.6 (.beta.-spodumene) and zirconium tungstinate. It is to be further
understood that materials other than ceramic can possibly be used if they
have a negative coefficient of thermal expansion and if they can satisfy
the stiffness and stability requirements of tube 62. An example of such
other materials is a carbon filled polyester liquid crystal polymer.
While a preferred embodiment of the present invention has been illustrated
and described in detail in the drawings and foregoing description, it will
be recognized that many changes and modifications will occur to those
skills in the art. It is therefore intended, by the appended claims, to
cover any such changes and modifications as fall within the true spirit
and scope of the invention.
Top